Vertical continuous casting devices of the aforementioned type are known, for example, from the Aluminum-Taschenbuch, 14th edition, p.22 ff. The mould consists of a low, water-cooled ring or annular mold cavity enclosure which, before casting begins, is closed by a base piece or die secured to the lowerable casting table. The metal flows in from the furnace at a low temperature through a channel into the die and, when it begins to solidify, the table is lowered and the emerging billet or bar is cooled directly by being sprayed with water. Reference is also made to U.S. Pat. Nos. 4,157,728 and 5,170,838.
When the lower edge of the cast billet reaches the region of secondary cooling, the corners of the billet base shrink and deform or curve upwardly away from the die. The extent of such deformation increases as a function of the side ratio and the shape of the billet. As a result of such deformation, the billet loses some of its stability on the die. Water runs into the gap between the die and the billet, evaporates and leads to "bumping". As a result of its reduced stability, the billet may wobble and become lopsided. Furthermore, the gap causes the thermal contact between the die and the lower end of the billet to be lost.
Under unfavorable conditions, the billet may melt or break open at its lower end, and metal may flow out, which, from a safety point of view, leads to a critical casting situation. Furthermore, as a result of the deformation on the narrower side of the billet in the mould, the surface layer which had formed there is lifted off the cooling running face of the mould, surface layer growth is disturbed and, under disadvantageous conditions, the surface layer may break open and melt, with melt then moving downwards and escaping. This may lead to a critical or dangerous casting situation and so-called icicles adversely affecting further processing of the billet may form on the narrower side of the billet. Said billet base deformation also increases the amount of billet base scrap, i.e. the part of the billet which has to be sawn off the lower end of the billet. Deformation is usually asymmetric, which further increases the amount of billet base scrap and the likelihood of the above defects occurring.
There exists a number of prior art measures attempting to reduce stresses in the billet base when casting begins, and thus the amount of billet base deformation.
A. T. Taylor et al. (Metal Progress, 1957, pp 70-74) have used compressed air to reduce the effect of secondary cooling when casting begins and thus to reduce the stress build-up in the case of billets having large dimensions.
N. B. Bryson (Canadian Metallurgical Quarterly, 7, 1968, pp 55-59) proposes so-called pulse water cooling wherein, during the initial casting phase, the flow of cooling water is periodically interrupted. As a result, the billet surface may temporarily reheat and cooling stresses are not built up to the same extent, so that the extent of billet base deformation is reduced. In large systems, said method requires expensive, rapidly acting valves to enable the cooling water to be switched on and off quickly.
Furthermore, the rapid switching action may induce severe overloading in the-power lines.
H. Yu (Light Metals, AIME Proceedings, 1980, pp 613 628) attempts to influence the actual cooling process by dissolving gases, preferably CO.sub.2, in water. When hitting the hot billet, the gas forms a thin insulating steam layer which reduces the rate of cooling, thus reducing the stress build-up and billet base deformation. However, the solubility of CO.sub.2 in water greatly depends on the starting temperature and the composition of the water. A specific adjustment of the cooling effect i.e. metering the amount of CO.sub.2 to suit the water quality can only be achieved by expensive measuring processes.
F. E. Wagstaff (U.S. Pat. No. 4,693,298) makes a similar proposal by suggesting that shortly before hitting the billet, the cooling water should be mixed with air while still in the mould. The air bubbles in the water are intended to function in the same way as the dissolved CO.sub.2. This method is known under the name of TurboCRT (Curl Reduction Technology). As far as the specific adjustment of the cooling effect as a function of water quality is concerned, it is subject to similar restrictions as the CO.sub.2 --method. Furthermore, distributing the air uniformly in the water presents a problem.
All the above methods when applied under practical casting conditions require a great deal of sophisticated technical equipment. Furthermore, they cause a considerable amount of additional maintenance expenditure and additional costs for providing CO.sub.2, and further costs result from the provision and consumption of energy for the purpose of generating compressed air.